Shape memory alloys (SMA) are a class of metallic materials, which undergo a reversible, diffusionless phase transformation. Roughly equiatomic Ni—Ti alloys are probably the most well known of these shape memory alloys. They are used in a wide variety of applications, including medical applications, such as stents used to hold blocked arterial passages open, heat sensing valves, robotic manipulators and undersea pipeline couplings. The mechanism by which these alloys appear able to remember and return to their original state prior to deformation is now reasonably well understood. At low temperature, martensite is the stable phase and will form spontaneously when cooled to a sufficiently low temperature. At high temperature, the austenite phase is stable and will form upon heating. When deformed beyond the elastic limit at intermediate temperatures, deformation occurs not by dislocation generation and motion, but rather by a series of small displacements on the atomic scale leading to the formation of stress-induced martensite. Upon heating to a sufficiently high temperature, the martensite reverts back to the parent austenite structure, leading to a reversal of the inelastic deformation and consequently, a return to the sample's original size and shape. This cycle of inelastic deformation at low temperature caused by some externally applied force, followed by reversion to austenite and the component's original shape upon heating is known as the one-way shape memory effect. The cycle can be repeated many times, depending on the extent of the deformation imposed per cycle. Deformations as high as eight percent are commonly reported for nickel titanium SMA's. Other SM materials are also emerging with larger strain capabilities and/or different use temperatures. Some utilize magnetic fields to effect shape recovery.
Ni—Ti SMA's can also be subjected to a thermomechanical treatment allowing them to exhibit a two-way effect in which one shape is obtained upon cooling and another pre-determined shape upon heating. The two-way effect is more limited however, to smaller recoverable strains, a lower number of cycles over which the effect is undergraded, and by the very low (subfreezing) temperatures typically needed to complete the low-temperature shape change.
Nickel titanium alloys that exhibit the SMA have attracted interest as actuators for distorting the shape of structures in a controllable way. Such structures are known as smart or intelligent structures. In addition to SMA-actuated structures, other types of actuators have been used, including pneumatic or hydraulic pressure, piezoelectric thin films and materials whose large coefficient of thermal expansion causes them to undergo relatively large strains during heating and cooling.
Reversible shape distortions can be achieved by using pairs of opposing pneumatic or hydraulic actuators working antagonistically. Efforts to achieve this with SMAs has been more problematic. Current reversible devices achieve their reversibility from the use of a some e sort of biasing element. That is, an elastic spring or tendon or some other elastic restoring agent that acts against the shape memory alloy is needed since typically once the shape memory alloy is allowed to cool it will not otherwise revert back to its original position. Accordingly, a biasing applies an opposing force to the SMA force. This is clearly undesirable since the actuators out put work is split into the work to change the shape of the structure and the work supplied to the bias element. An advantage of the present invention shape memory multifunctional structure is that it does not require any bias element. For example, neither springs or other elastic restoring devices are required. Because more of the available work is available to distort the structures, a smaller actuator can be used and therefore the mass of the system proposed here can therefore be less than that of competing concepts.
Lu et al. (See Lu, T. J., Hutchinson, J. W. and Evans, A. G., “Optimal design of a flexural actuator”, J. Mech. Phys. Solids 49 (2001), 2071-2093) discusses a shape-changing sandwich panel in which a metallic or polymer truss core is placed between shape memory alloy face sheets, which are integrally bonded to the core. However, the core does not exhibit any shape memory effect and consists of a single layer of triangular cells whose areas are defined by the (straight) truss members and the Lu et al. panel is reversible through a two-way shape memory effect only. Whereas the present invention shape memory multifunctional structure provides an actuating sandwich panel capable of fully reversing its change in shape to return to its initial configuration by virtue of SMA face sheets exploiting a one-way shape memory effect only.
Therefore, there is a need in the art for a reversible shape memory structure that has a way of sensing or responding to some stimulus or change in its environment as well as satisfy structural demands of the given application with minimal use of material. Such a material is bi-functional (i.e. it supports loads and changes shape). It is an example of a new class of so called multifunctional materials.